1. Field of the Invention
This invention relates to the production of microfilm and microfiche devices, and particularly to the production of microfilm and microfiche records which can be updated. For the sake of simplicity, microfiche devices will be referred to throughout the following description, but it will be apparent that the invention applies equally to the production of microfilm records and other imaging products.
2. Description of Related Art
Microfiches are optical storage media on which very large amounts of data can be stored on a small flexible sheet. For example, a standard A6 size microfiche can store the equivalent of ninety-eight A4 pages of data, arranged in an array of columns and rows. It is quite usual for many copies to be made from a master microfiche, for example where microfiches containing catalogues of books are distributed to libraries.
Such microfiches are usually produced by a photographic process, and because the whole of the photosensitive sheet is developed and fixed in the process, no further data can be subsequently added to the record. It will be apparent, therefore, that if the data requires updating, a new master must be produced and all of the copies must be discarded and new copies made.
A number of updatable microfiches are available. These rely on, for example, electro-photographic processes, or recording on thermoplastic materials, or a combination of dry silver films and thermal processes. Copies of such microfiches are at present made on silver-based photographic sheets or on diazo films, or vesicular copies are made. The copying process may retain the original image polarity or may invert it, depending upon which medium is used for the copies. For example, a positive contrast image will produce a negative copy on a silver-based photographic film. In each case, the copy cannot be altered once it is made, so new copies must be produced each time the master microfiche is updated. The copying process will, in many cases, involve the use of wet chemical processes.